Descartes was the first philosopher to address the problem of mind from within the new mechanistic worldview, which would later be developed by Newton as the foundation of classical science. This mechanistic view poses an intrinsic problem since it does not seem to leave any space for mental phenomena. Descartes solved this problem by proposing two independent realms: mind and matter. While matter follows the laws of mechanics, mind has a logic of its own that cannot be reduced to mechanical principles. This philosophy is known as dualism. It is mostly outdated, although some philosophers and even brain scientists still hold on to it.
The assumptions of dualism are simple. Outside, we are surrounded by material reality. This consists of hard, indivisible particles or pieces of matter, which obey the deterministic, mechanical laws of nature. Such determinism leaves no place for free will, intention or agency: since all material events are already fully determined by the laws of nature, there is no freedom to intervene or change the course of events. The atomic structure of matter leaves no place for thoughts, feelings, consciousness, purpose, or other mental phenomena. Therefore, we need to assume that there exists another reality inside: the mind, which reflects about external reality as perceived through the senses. Descartes conceived this mind as an immaterial soul, having a free will. To explain how this mind could still affect the body, which obviously is made out of matter, he assumed that the mind communicates with the body through the pineal gland, a small organ in the brain stem.
While simple and intuitive, dualism creates a number of fundamental problems. First, adding the independent category of mind to the one of matter obviously makes things more complicated. More fundamentally, as pointed out by the 20th century philosopher Gilbert Ryle, Descartes' mind functions like a “ghost in the machine” (similar to the Deus ex Machina that suddenly drops from the sky to solve all problems when the plot in a novel or play has become too complicated). The body behaves like a mechanical, deterministic machine. Yet, it is inhabited by some spooky “ghost” that pulls the strings, and that performs all the tricks that are too complicated for us to understand mechanically. Indeed, we have no scientific theory of mind as a separate category, unlike our very reliable and precise theories of matter.
Finally, if mind can affect matter beyond what matter would already do on its own, then it must contravene the deterministic laws of mechanics, implying that these otherwise very reliable laws cannot be trusted. In spite of these shortcomings, Descartes' dualist philosophy remains simple and intuitively attractive. It is still (implicitly) used nowadays by scientists and lay-people, albeit most often in a “materialist” version where the mind is replaced by a hypothetical homunculus